1. Field of the Invention
The present invention relates to a magnetic recording medium, and in particular to a perpendicular magnetic recording medium for use in any of various magnetic recording devices such as an external storage device for a computer.
2. Description of the Related Art
To increase recording density in magnetic recording, the perpendicular magnetic recording method is attracting attention as an alternative to the conventional longitudinal magnetic recording method. This is because, compared with the conventional longitudinal magnetic recording method, the perpendicular magnetic recording method has advantages in that there is high thermal stability at a high recording density, and writing can be carried out sufficiently even with a recording medium having a high coercivity, and hence the recording density limit for the longitudinal magnetic recording method can be exceeded.
With a perpendicular magnetic recording medium, to record information with the direction of magnetization being perpendicular to the film plane of the magnetic recording layer, the magnetization must be stably maintained in the direction perpendicular to the film plane. The magnetic recording layer used in such a perpendicular magnetic recording medium is thus required to have a high perpendicular magnetic anisotropy constant (Ku value). The Ku value of the magnetic recording layer in perpendicular magnetic recording media currently being studied is approximately at least 1×106 erg/cm3.
With magnetic grains having uniaxial magnetic anisotropy, the magnitude of magnetic field required to reverse the magnetization is called the anisotropy field Hk, and in general Hk is expressed in terms of the saturation magnetization Ms and the Ku value as Hk=2Ku/Ms. To bring about magnetization reversal, a magnetic field greater than Hk is required, and this value is proportional to the Ku value. With a magnetic recording medium, if Hk is too high, then magnetization reversal upon writing using a magnetic head will thus being sufficient, and hence proper operation will no longer be possible; a suitably moderate Hk value is thus required.
With a magnetic recording medium which is an aggregate of magnetic grains, the average magnetization reversal field, which is called the coercivity Hc, is determined by the distribution of the axes of easy magnetization and the Hk values of the individual magnetic grains, and the strength of magnetic interactions between the magnetic grains and soon. In the case that magnetic interactions between the magnetic grains are small, the Hc value approaches the Hk value.
Moreover, the energy barrier E that must be surmounted to reverse the magnetization is given by E=KuV(1−H/Hk)2, where H is the magnetic field applied in the direction of the axis of easy magnetization, and V is the grain volume. If this energy barrier E is not sufficiently high relative to the thermal energy kBT (kB is Boltzman's constant, T is the absolute temperature), then the magnetization will reverse under the influence of thermal energy. This is called thermal fluctuation (or thermal disturbance) of the magnetization, and implies loss of information on the magnetic recording medium; the value of KuV, which determines the energy barrier E, must thus be kept relatively high. Moreover, even if thermal fluctuation of the magnetization does not lead to loss of information, thermal fluctuation will surface as medium noise called reverse magnetic domain noise caused by partial reversal of recorded bits.
Note that KuV/kBT is generally used as an indicator of thermal fluctuation, but this assumes that an external magnetic field is not being applied; an indicator of thermal fluctuation when a magnetic field H is being applied uses the energy barrier E described above, and is thus KuV(1−H/Hk)2/kBT.
Furthermore, to reduce the medium noise and thus improve the quality of recorded information signals, i.e. to improve the signal-to-noise ratio (SNR), it is necessary to reduce the value of the activation grain size D=V/δ (here, δ is the thickness of the magnetic recording layer), i.e. make the units of magnetization reversal small. In the case that the units of magnetization reversal are small, minute recorded bits can be properly written, and hence the SNR is improved. Many studies have thus been carried out into reducing the value of D with perpendicular magnetic recording media. To reduce the value of D, it is effective to reduce the crystal grain diameter in the magnetic recording layer, and moreover reduce magnetic interactions between the crystal grains.
From the above, when the value of D is lowered to improve the SNR, the value of V drops, and hence a high Ku value becomes necessary to maintain the value of the energy barrier E required to keep the magnetization stable. On the other hand, in the case that the Ku value is kept high, the Hk value increases, i.e. the magnetic field required to reverse the magnetization increases, and hence writing of information with a magnetic head becomes difficult. That is, with a magnetic recording medium, it is very difficult to satisfy all of 1) improving the SNR, 2) making the magnetization thermally stable (decreasing the reverse magnetic domain noise), and 3) making writing with a magnetic head easy, and there is a trade-off between these three factors.
As perpendicular magnetic recording media the aim of which is, out of the above three factors, to both improve the SNR and make the magnetization thermally stable, there have been proposed perpendicular magnetic recording media having so-called functionally separated type magnetic recording layers in which a plurality of magnetic recording layers having different Ku values are formed on top of one another (see, for example Japanese Patent Application Laid-open No. 11-296833 and Japanese Patent Application Laid-open No. 2000-76636).
In Japanese Patent Application Laid-open No. 11-296833, it is disclosed that by forming on top of one another a layer of a region having a high Ku value so that the thermal stability of the magnetization is high (upper layer) and a layer of a region having a somewhat low Ku value so that magnetic interactions between the crystal grains are small and hence the SNR is high (lower layer), a medium having high thermal stability of the magnetization and a good SNR can be produced. Note that in an embodiment, it is disclosed that the Ku value of the upper layer is made to be 2.5×106 to 5×106 erg/cm3, and the Ku value of the lower layer is made to be 1×106 to 2.5×106 erg/cm3.
Moreover, in Japanese Patent Application Laid-open No. 2000-76636, a similar technical idea is disclosed, with it being disclosed that magnetic recording layers having different Ku values and crystal orientations are formed on top of one another, whereby similar effects are obtained.
However, the matters disclosed in Japanese Patent Application Laid-open No. 11-296833 and Japanese Patent Application Laid-open No. 2000-76636 relate to simultaneously improving the SNR and making the magnetization thermally stable, but no consideration is given to the ease of writing with a magnetic head.
With increasing recording densities, there is an ever strengthening need to maintain a high Ku value and reduce the D value so that small recorded bits can be stably maintained, and with such a medium, it is very important to secure ease of writing with a magnetic head.